Like the rest of the body, the skin ages. This cutaneous aging is manifested more particularly in the appearance of wrinkles, brown spots, etc, which are the visible consequences of disturbances to the cell metabolism and to the organization of the extracellular matrix.
The cause of this aging is both biological and environmental.
In its role as a barrier to external attack, the skin is subjected to major environmental stresses, and more particularly to oxidative stress.
In order to control this oxidative stress, natural systems of regulation and protection exist that are either enzymatic in nature (superoxide dismutases, catalase, peroxidases) or else nonenzymatic in nature, such as low molecular weight antioxidants, which are, for example, vitamin E, vitamin C, and glutathione.
These antioxidant species may be found in the surface layers of the skin.
At the surface of the skin, its antioxidant species, i.e., reductive species, react with the oxidative species that attack the skin. The quantity and the nature of these oxidative and reductive species present on the surface of the skin determine the redox state of the skin.
It is therefore advantageous to be able to analyze these species qualitatively and to quantify them in order to gain knowledge of the mechanisms of control of oxidative stress and to have an evaluation of the state of skin aging.
The measurement of reductive species at the surface of the skin has already been studied by electrochemical methods:                either by indirect measurement: an extraction solution is contacted with the skin for a predetermined time and then the oxidoreductive species are assayed by cyclic voltammetry (Kohen et al. (1999) Methods in Enzymology, 300, 428-437),        or with a set of a number of electrodes which are contacted with the skin (U.S. Pat. No. 6,108,570 by Kohen et al.; or FR 2 845 264, which describes a “device and method for direct measurement of pH and of oxidation state” by means of a working electrode and an auxiliary electrode, which may be grouped together either in the same capillary or in two concentric capillaries, a reference electrode being used, furthermore, independently and separately).        
These prior art devices either do not allow direct measurements or else are not simple to employ, given that they require the application of different electrodes to the skin. Furthermore, they are applied to very small skin surface areas.
There is therefore a real need for a device which is simple to use and allows effective control over the operating conditions and hence good reproducibility, and which allows homogeneous measurement over a representative portion of the skin. This device shall, moreover, allow continuous and noninvasive measurement of the redox state of the skin.